In the wet mechanical processing of material mixtures, e.g. waste, mechanically removed waste fractions or commercial residues, slurry is produced, e.g. pulps or suspensions, which still contain relevant amounts of materials that can sediment in water or that have sharp edges, e.g. gravel, grit, stones, ceramic or glass fragments or metal particles, which cause operating problems, e.g. deposits or wear, in downstream process stages. This results e.g. in layers of sediment in containers which necessitate laborious emptying after a few years of operation, the laying of pipelines that require a large cleaning effort, or a large degree of wear of the machine technology caused by the mostly abrasive properties of these materials.
Organic waste suitable for fermentation may contain mineral dense materials of 4% by weight (Kübler, H., Hoppenheidt, K., Hirsch, P., Kottmair, A., Nimmrichter, R., Nordsieck, H., M., Mücke, W., Swerev (2000) Full scale co-digestion of organic waste. Water Science & Technology 41, 195-202). Communal bio-waste contains relevant amounts of mineral dense materials such as stones, glass fragments, grit, gravel or sand which, according to the studies carried out by Kranert et al. (Kranert, M., Hartmann A., Graul S. (1999) Determination of sand content in digestate. In: W. Bidlingmaier et al. (editor) Proceedings of the International Conference ORBIT 99 on Biological Treatment of Waste and the Environment, Part I, pages 313-318) can make up a portion of the dry mass of the waste of partially over 25% by weight. During wet mechanical processing of the bio-waste a substantial part of these mineral dense materials is carried into the pulp which is then taken for biological recycling. Studies carried out by Kübler et al. (Kübler, H., Nimmrichter, R., Hoppenheidt, K., Hirsch, P., Kottmair, A., Nordsieck, H., Swerev, M., Mücke, W. (1998) Full scale co-digestion of biowaste and commercial organic waste. Materials and Energy from Refuse. P. De Bruycker and J. Kretschmar (editor), Techological Institute of Antwerp, pages 195-202) show that in the wet mechanical processing of bio-waste a pulp is produced from which hydrodynamic separation of dense materials removes approx. 3% by weight of the moist mass of the waste being treated as dense materials.
During the operation of waste treatment plants in which the sieved out fraction of less than 80 mm is taken for wet processing, in this fraction a portion of glass particles and mineral components of 12 to 14% by weight was determined in the moist mass of this fraction (Rita, J., Braga, J., Mannall, C., Goldsmith, S., Kübler, H., Rahn, T., Schulte S. (2015) Compost-like material or thermal valorisation—impact on MBT Plant economics and environmental aspects—case studies in Portugal and UK. In: M. Kühle-Weidemeier and M. Balhar (editor) Energy and raw materials from residue and bio-waste, Cuvillier Verlag Gottingen, pages 395-406).
In order to ensure disruption-free recycling of the slurries or suspensions from the wet processing, the easily sedimentable portions are often removed from the suspension. For this purpose dense material separators are used. In addition to the removal of the extraneous materials, these dense material separators must however also minimize discharge of the other components present in the slurry and should be recycled in the downstream process stages, e.g. fermentable organic materials. This can be achieved by a combination of a hydrocyclone and a classifying tube which is disposed in the lower section of the hydrocyclone for the discontinuous discharge of the dense materials that have been removed. In order to reduce the discharge of the other components, flushing liquid is often delivered to the classifying tube. In this way a counter-flow is generated in the classifying tube which releases the dense materials that have been removed from the other components of the slurry.
This type of apparatus is described in DE 195 05 07 A1, which apparatus has a flat bottomed hydrocyclone for the removal of dense materials from a slurry which was generated from waste materials. A classifying tube is positioned downstream of the flat bottomed hydrocyclone in order to increase the selectivity of the dense material separator. The dense materials that are removed are collected in the lower section of the classifying tube by means of a sluice system with an integrated chamber and are discontinuously discharged. If subsequently to emptying the chamber the shut-off valve to the classifying tube is opened, the content of the classifying tube and part of the content of the hydrocyclone is emptied all at once into the chamber. On the other hand it may be that the dense materials located within the chamber cement and so make it difficult, if not actually prevent, discharge from the chamber. In this way the zone for the selective removal of dense materials is disrupted and the selectivity of the separation result is worsened. In said document reference is also made to the fact that the cleaning effect of the classifying tube is improved if a flushing liquid is delivered to the classifying tube against the pressure that prevails in the hydrocyclone and is discharged via the upper section of the cyclone. Tap water or some other liquid is provided as the flushing liquid.